The invention relates to a device and a method for the safe threshold-detection of state information from an analog signal, intended in particular for a railway type application.
Such a device is used for the acquisition of reliable information on the states of a track circuit, for example of a light or a shunt position, by a safety computer which then executes processes associated with the change of state of the information and then sends commands to signaling equipment.
A state of a command or signaling variable is known to be referred to as a “restrictive state”, which state, when it is detected and when it is incorrect, leads to serious danger of an accident.
A state of a command or signaling variable is known to be referred to as a “permissive state”, which state, when it is detected and when it is incorrect, does not lead to a loss of safety.
The term “safe detection” defines the design of a device or a method which ensures that it is impossible to detect a permissive state on a signal representing a restrictive state. Conversely, in such a design, the detection of a permissive or restrictive state on a signal representing a permissive state is allowed.
Conventionally, a safe detection circuit comprises a safe threshold circuit implemented in the form of a diode or a transistor, and which uses in a safe manner the non-linear variation in impedance of the semiconductor junction present in these components.
The design and safety validation of such a detection circuit is extremely complex and costly due in particular to an ill-defined switchover limit in the linear zone.